1. Field of the Invention
The present invention relates to thin film composite electrolyte structures that are preferably ionically conductive and are therefore suitable for use in electrochemical cells, such as secondary batteries based on sodium and sulfur.
2. Related Art
Solid, ionically conductive electrolyte components are utilized in high temperature electrochemical cells, such as secondary batteries based on sodium and sulfur or sodium and a metal chloride. Such electrochemical cells are typically comprised of: a) a liquid anodic reactant; b) a liquid cathodic reactant; and c) a solid electrolyte component that separates the cathode from the anode and that is permeable by either ions from the anodic or cathodic reactants. For example, when the cell is a sodium-sulfur cell, the anodic reactant is liquid sodium, the cathodic reactant is liquid sulfur or a mixture of sulfur and sodium polysulfides, and the electrolyte component is typically comprised of materials such as beta double prime alumina ((β″-Al2O3) or (Na2O.5Al2O3)), NASICON (Na3Zr2Si2PO12) or other materials that are permeable only by sodium ions. A complete description of the fabrication and operation of sodium-sulfur cells can be found in the publication by J. L. Sudworth and A. R. Tilley entitled “The Sodium-Sulfur Battery” (Chapman and Hall, New York, 1985). The cell reaction that produces power (discharge cycle) in this device is most generally given as:2Na+xS→Na2Sx 
During the discharge cycle, sodium is oxidized (i.e., gives up an electron to form a sodium cation) at the anode and sodium ions migrate from the anode compartment through the solid electrolyte. Therefore, the reaction at the anode during discharge is:2Na0→2Na++2e−
Simultaneously, the following reaction takes place at the cathode where elemental sulfur is reduced:S0+2e−→S=The flow of electrons through the external circuit from the anode to the cathode produces power to drive equipment such as electric motors.
A critical component of the sodium-sulfur battery is the solid electrolyte that allows only the transport of sodium cations while blocking the transport of sulfur anions. Since the original conception of the sodium-sulfur battery, the electrolyte of choice has been β″-Al2O3 (Na2O.5Al2O3). Other materials, such as NASICON, have shown promise.
There are certain important requirements that the electrolyte component must meet in order to perform effectively in a high temperature electrochemical cell. One of these is a high ion flux; and because ion flux is inversely proportional to the thickness of the electrolyte component, it is desirable to make the electrolyte layer as thin as possible. Another important requirement is low electronic conductivity and this is governed by the choice of electrolyte materials.
There are several conventional methods for fabricating such electrolyte components. For example, there are teachings describing the fabrication of solid electrolyte structures (typically tubes or plates) from β″-Al2O3 powder and assembly of the resulting tubes or plates into a structure that is typically hermetically sealed for use in automotive and electrical utility load leveling applications. While β″-Al2O3 tubes can be prepared by isostatic compression of the powder, the preferred method of fabrication is electrophoretic deposition as described in U.S. Pat. Nos. 3,896,018; 3,896,019; 3,900,381; and 3,976,554 (all of which are incorporated herein by reference). Further, U.S. Pat. No. 4,038,464 (incorporated herein by reference), discloses the use of fibrous mats in both electrode compartments to enhance the conductivity of the electrodes. The fabrication of β″-Al2O3 in shapes other than tubes for greater cell efficiency is taught in U.S. Pat. Nos. 4,226,923; 4,568,502; 5,053,294; and 5,112,703 (all of which are incorporated herein by reference).
There are several inherent disadvantages to cells made by conventional methods. One disadvantage is that electrolytes made from materials such as β″-Al2O3 have low ion flux, because the electrolyte must be thick enough to also provide mechanical support. So, while it is generally known that thinner electrolytes have higher ion flux, these unsupported electrolyte tubes of the prior art can typically not be optimized for high flux because of the limitations of their mechanical strength.
Another disadvantage of conventional methods results from forming the electrolyte as tubes on mandrels (typical of the electrophoretic deposition method) because there is a limit on the minimum diameter of the tube and also on its maximum length. This, in turn, limits the surface area to volume ratio, and thus the energy density, of the resulting electrochemical cell. For example, sodium-sulfur cells made by conventional techniques have relatively low surface area to volume ratios; and, consequently, they have lower energy densities than desired.
One approach to mitigating these disadvantages is to support the solid electrolyte as a thin film on a suitable microporous support. U.S. Pat. No. 4,244,986 (incorporated herein by reference), discloses the application of a precursor to β″-Al2O3 (in the form of a solid) onto supports such as α-alumina, mullite or zirconium oxide. One concern with respect to this approach is the difference in the coefficient of thermal expansion (CTE) between β″-Al2O3 (8.6×10−6/° C.) and the preferred supports a-alumina (8.2×10−6/° C.), zirconium oxide (8.2×10−6/° C.) and especially mullite (5.2×10−6/° C.). A second concern is that none of the listed refractory ceramic supports are electrically conductive, thereby limiting the configuration of a sodium-sulfur cell made by this process to one in which the anode reactant is on the support side.
While U.S. Pat. No. 4,526,844 (incorporated herein by reference) discloses that NASICON can also be used as the solid electrolyte in a sodium-sulfur cell, there are no actual examples showing that a thin film composite of such a material was ever made. This may be because of the significant mismatch in CTE between NASICON (˜1×10−6/° C.) and most microporous substrates.
U.S. Pat. No. 5,059,497 (incorporated herein by reference) discloses the fabrication of a composite, ion-conductive electrolyte member comprised of a first layer of an ion conductive material such as β″-Al2O3, and a second, or substrate layer, comprised of a material selected from aluminum silicon carbide, doped tin oxide, graphite, or composites, compounds, mixtures, and/or combinations of these materials. A preferred material is selected from the titanium dioxide family as disclosed in U.S. Pat. Nos. 4,422,917 and 3,985,575 (both of which are incorporated herein by reference) (tantalum or niobium-doped TiO2). According to the '497 patent, the first layer is much thinner than the second in order to provide higher ion flux and the second layer is substantially thicker to provide suitable mechanical support. The first layer can be applied by, among other techniques, electrophoretic deposition.
U.S. Serial No. 20020172871A1 (incorporated herein by reference) describes one method of fabricating an improved electrochemical cell a) with an anode (and corresponding anodic reactant); b) a cathode (and corresponding cathodic reactant); and c) a composite ion-conductive electrolyte structure with: i) a first layer, preferably thin-film in nature, that is a mixture of two or more chemically distinct compounds, at least one of which is ion-conductive; and ii) a second layer bonded the first layer, where the second layer is any refractory support structure having: (1) an effective porosity that will allow an effective flow of anodic or cathodic reactants to the first layer; and (2) a coefficient of thermal expansion within 5% of the first layer's material. The objective of utilizing a mixture of two or more chemically different compounds in this application was to achieve a match in the coefficient of thermal expansion.
Another attempt to construct a suitable solid electrolyte is described in U.S. Pat. No. 3,901,733 (incorporated herein by reference). This patent describes a two layer beta alumina structure. The first layer is a thin (50-1,000 microns) conductive dense layer of β alumina, and the second layer is a thicker substrate or backing of porous beta or beta double prime alumina. The conductive dense layer is coated on the substrate (plasma or slurry spraying, or granular deposition), and sintered to a final temperature between 1,500 to 1,800° C. Unfortunately, such an asymmetric two layer composite is subject differential shrinkage in the sintering process, making the composition unreliable as a structural material.
Therefore, while conventional methods have provided solid ion-conductive electrolyte components for sodium sulfur electrochemical cells with varying commercial success, there is still a need in the art for solid ion-conductive electrolytes with improved ion-conductive properties and improved mechanical properties that can withstand the stresses associated with wide temperature swings. There is also a need in the art for improved methods of making these solid ion-conductive electrolytes. The present invention addresses these needs.